1. Field of the Invention
The present invention relates to optical print heads and to means for transmitting light emitted from the print heads to a photosensitive surface, and more particularly to means for accurately aligning the transmitting means and the print head to maintain high resolution and good exposure uniformity.
2. Description of the Prior Art
High speed optical printing devices used in data processing systems are intended for converting electric input signals into printed form, and include photosensitive means and light sources such as linear arrays of light emitting diodes (LEDs), photodiodes, or similar devices. Light from a source is typically transmitted to the photosensitive means via a plurality of gradient index optical fibers forming an imaging lens array. Such imaging lens arrays are commercially available as Selfoc (a trademark of Nippon Sheet Glass co., Ltd.) lenses in a staggered two-row bundle of optical fibers as described in U.S. Pat. No. 3,658,407, patented on Apr. 25, 1972. Reference may be made to that patent for details of the lens arrays.
In printheads of the type referred to herein, the LEDs are arranged in a linear array of LEDs having a designed density to provide a resolution of a predetermined number of dots per inch. In addition, they have a Selfoc lens assembly supported over the LED array to provide a modular unit which can be prefocused and mounted upon a suitable reproduction apparatus. These lenses comprise a plurality of fiber optic filaments bundled together to from a linear array having dimensions approximating the dimensions of the LED array. Because of the preciseness required in high resolution printheads of both the LED array and the lens assembly, special mounting precautions must be taken to insure stress and distortion-free performance of the printhead over a wide temperature range.
One requirement in such printheads is to maintain the lens assembly as straight as possible. U.S. Pat. No. 4,715,682 illustrates one method by which the lens assembly can be kept straight. According to that patent, a linear fiber optic imaging lens array is mounted on the print head and fixed in place after alignment of the array in the Y and Z directions. As noted in the patent, the coefficient of thermal expansion of the imaging lens array and stiffeners attached thereto may differ from that of the printhead, and must be accounted for in the design of the mounting means. To this end the patent suggests that the printhead provide for thermal expansion after the imaging lens array has been properly aligned with and spaced from the LED array. This is accomplished by use of a flexible plate against which an arm of one of the stiffeners can expand against to thus allow expansion in the X direction of the array while movement in the Y and Z directions is constrained.
A problem with this approach is that where the temperature coefficient of the stiffener bar is different from the printhead there are different thermal expansions in say the Y direction that are not relieved and this can cause undesired distortion of the lens array.
It is therefore an object of this invention to provide an improved support for the linear lens array that overcomes thermal problems associated with supports known on the prior art.